This invention relates to inks that exhibit different visible properties dependent on external light conditions.
TCS INDUSTRIES in North Carolina (not admitted to be prior art to the present invention) advertise black and white and color emblems on T-shirts which are said to exhibit their color only in the presence of the sun's ultraviolet rays. The photochromic inks are said to react with the sun's ultraviolet rays and thus will not cause unsightly body heat and perspiration shadings associated with other changeable inks.
Trundle et al, U.S. Pat. No. 4,927,180 describe a photochromic ink, containing a photochromic fulgide, fulgimide or spiropyran, which reversibly convert to a colored form upon exposure to U.V. light. A superficial protective layer is provided to protect the photochromic compound from moisture and oxygen.
Hawkins et al., U.S. Pat. No. 4,992,347 describe photochromic compounds, specifically fulgides, that can be permanently converted into non-photochromic compounds by overexposure to U.V. light.
Brettle et al., U.S. Pat. No. 4,812,171 describe a photochromic marking, containing 1,2-dihydroquinoline, which is irreversibly converted to a colored state by exposure to U.V. light.